Radial inflow gas turbine engines must be designed for light-weight turbine wheel containment. This requirement of containment may impose upon the design constraints required for a combustor, specifically because a small diameter containment ring is located between a turbine nozzle shroud and a combustor inner liner to contain turbine blades in case of mechanical failure due to excess temperature or wear. The containment ring must be kept relatively cool to retain high performance material properties and containment capability. To protect the containment ring from hot combustion gases in the combustor, a heat shield may be employed as an extension of the combustor inner liner. This heat shield seals against the turbine nozzle shroud near its outer diameter while film air cooling may be used to maintain acceptable heat shield operating temperatures. A circumferentially uniform seal and low heat shield temperatures are imperative for high combustor performance and extended engine life.
Problems associated with employing a heat shield in the design of a gas turbine engine are twofold. First, differential growth between the heat shield and the turbine nozzle shroud during thermal transients can plastically deform the heat shield. Such deformation, which is aggravated by a relatively large seal diameter, may degrade the seal between the heat shield and the turbine nozzle shroud. Secondly, combustor performance design requirements limit an amount of air flow available to cool the heat shield. In most combustor designs, a significant amount of air flow cannot be tolerated as pure leakage.
The desirability of optimizing design while providing adequate cooling for a containment ring is recognized in a patent to Shekleton, U.S. Pat. No. 4,955,192, in which a dilution air path is employed. Small openings may be included in a gas turbine to inject air into a combustion annulus to produce a localized air film on inwardly facing surfaces of inner, outer, and radially extending walls which define the combustion annulus. As a consequence, a containment ring may be located at a radially inward position to minimize its mass and may be adequately cooled to allow use of nonexotic materials in fabricating the same. However, the patent to Shekleton does not, as the invention to be described more fully hereinafter, disclose an inner slotted surface of an inner combustor wall which abuts to an inner surface of an annular lip of a rear turbine nozzle shroud to accommodate thermal transients between the same, while simultaneously providing localized air film cooling of the inner slotted surface acting as a heat shield for a containment ring.
The problem of decreased operational efficiency and potential mechanical failure caused by significant thermal growth from extremely high temperatures on a turbine side of an engine is also discussed in a patent to Harris et al, U.S. Pat. No. 4,932,207. The Harris patent teaches an improved seal plate whereby the clearance between the seal plate, which separates compressor and turbine sections, and a turbine may be minimized to reduce performance losses. The clearance is minimized by forming a seal assembly in part out of a plurality of segments disposed in a circular array which are relatively movable but sealed to each other. However, the patent to Harris does not, as the invention to be described more fully hereinafter, provide for a circumferentially uniform and sound seal to solve a thermal gradient problem between a heat shield and a rear turbine nozzle shroud on the turbine side of the engine by employing slots on an inner face of a combustor annulus which abut a rear turbine nozzle shroud.
An improved structural arrangement for a centrifugal compressor provided with an efficient peripheral diffuser arranged in a closely coupled relationship with a combustion chamber of a gas turbine is disclosed in a patent to Paul et al, U.S. Pat. No. 3,014,694. A means for sealing the gas turbine against leakage of motive fluid and providing for minimum axial loading of high speed bearings which support a turbine shaft is discussed. However, the patent to Paul does not address, as does the instant invention, a problem of accommodating large thermal differential expansion in a gas turbine between a heat shield and a turbine nozzle shroud while minimizing an amount of cooling air required to maintain acceptable heat shield temperatures necessary for heat shield material strength.